(1) Field of the Invention
The invention relates in general to loudspeaking substation systems and more particularly to loudspeaking substation instruments employing dual tone multi-frequency oscillators for dial signalling.
(2) Description of the Prior Art
Loudspeaking substation instruments, generally referred to as speakerphones are arranged both for "distant-talking" by means of a microphone and loudspeaker and regular or "close-range" communication by means of a handset transmitter and receiver.
Earlier models speakerphones, incorporating dual tone multi-frequency (DTMF) harmonic oscillators for dial signalling use a transmission network for tone regulation, as a function of loop length, in the handset mode and a separate stand-alone network for tone regulation in the speaker mode. The separate network consists of a resistor and varistor combination placed across the telephone line only during the signalling operation. This was accomplished by hookswitch contacts which electrically shorted the unused network.
Besides the obvious drawback of increased component count with the above mentioned arrangement, the output tone levels between the handset mode and the speaker mode differed due to the differences of separate loop regulating networks.
Accordingly, it becomes an object of the present invention to provide an improved speakerphone circuit which negates the requirement for a separate speaker mode loop compensation circuit for tone regulation when employing a DTMF dial signalling device.